What is the specific function of DNA ligase in genetic engineering?

• A. Copies DNA.
• B. Cuts DNA at specific sequences.
• C. Seals breaks in the sugar-phosphate backbone by forming phosphodiester bonds.
• D. Replicates DNA.

Answer: (C)

DNA ligase's function in genetic engineering is to seal breaks in the sugar-phosphate backbone of DNA by forming phosphodiester bonds. After fragments are joined—for example, an insert is ligated into a plasmid—ligase closes the nicks to create a continuous, covalently bonded molecule. This sealing step makes recombinant DNA stable and intact for propagation in cells. It’s not responsible for copying DNA (that’s done by DNA polymerases), nor for cutting DNA at specific sequences (that’s done by restriction enzymes). It also isn’t the enzyme that replicates DNA as a whole; ligase specifically closes the gaps to knit the assembled pieces into one complete strand.